This invention relates to a step of removing sulfur from a froth, said step being preferably embodied in a process for the removal of hydrogen sulfide from gas streams by means of a regenerable scrubbing solution adapted to preferentially oxidize the sulfur of the hydrogen sulfide to elemental sulfur. Thus, the invention particularly relates to a method of improving the separation of elemental sulfur from the aforementioned scrubbing solution, and especially to improving the characteristics of a flotation froth containing particulate sulfur so it can be readily dispersed in a slurry.
Various processes such as the Stretford process and the Takahax process have been developed for the removal of hydrogen sulfide from gas streams containing about 1 to 2 percent of hydrogen sulfide. A common feature of these processes is that the gas stream is contacted with an aqueous solution adapted to absorb the hydrogen sulfide and to oxidize the sulfide to elemental sulfur, whereafter the solution is regenerated in an oxidizer by bubbling air therethrough. The sulfur formed during the contacting step is removed from the solution in the oxidizer as a froth which is then processed further into a saleable product. The froth processing steps generally include a slurrying step where the froth is dispersed as a readily filterable slurry. The resultant slurry is then generally filtered by a conventional filter, e.g., a plate and frame filter.
The flotation of the elemental sulfur particles from the solution as well as the further handling thereof present certain difficulties. U.S. Pat. No. 4,508,537 describes a method of improving the removal of sulfur particles for the scrubbing solution of a Stretford process in which a non-ionic surfactant together with diesel oil are used in a ratio of one to the other of from 7:1 to 1:7. This combination of additives was claimed to have had a dramatic effect on the efficiency of the flotation step but no mention is made in U.S. Pat. No. 4,508,537 of the influence of the said additives on the ease of handling or manipulation of the froth obtained. In this regard, reference is particularly directed to the phenomenon of the tendency of the sulfur in the froth to become crusty and stiff, i.e., the froth is low in scrubbing liquor content. This results in the formation of lumps which are almost impossible to pump to the plate filter. This phenomenon is in contradistinction to the tendency of sulfur particles to fall out of the washing solution and deposit in the circulating equipment as described in U.S. Pat. No. 4,508,537. (The terms "scrubbing", "washing" and "absorbing" may be used interchangeably.)
It has been discovered by the present inventors that in a process in which diethanolamine has been eliminated in accordance with Republic of South Africa Patent No. 87/3112, problems of the kind described above are experienced with regard to the handling properties of the sulfur froth after separation thereof from the washing liquor, but, conversely, little difficulty was experienced with regard to the flotation of the elemental sulfur particles from the scrubbing solution. It became clear that, while the presence of diethanolamine under the process conditions previously applicable to the known process had no effect on the absorption and reoxidation stages, it did play the role of a wetting agent, albeit not a very efficient one. It therefore became desirable to investigate the use of alternative, preferably more cost-effective wetting agents to substitute for diethanolamine with a view to eliminate the problems encountered in the handling and manipulation of the froth described above.